1. Field of the Invention
The present invention relates to an electronic device and a method of manufacturing the electronic device and, more particularly, to an electronic device wherein a circuit chip is mounted on a film-like substrate and a method of manufacturing the electronic device.
2. Description of the Related Art
Conventionally, there is widely known an electronic device wherein a circuit chip is mounted on a substrate, such as a printed wiring substrate. This electronic device is used in such applications where it is built into an electronic apparatus to control the apparatus or is operated on a stand-alone basis to exchange information with an external apparatus. As an example of the electronic device, there are known various types of radio frequency identification (RFID) tags for contactlessly exchanging information with an external apparatus as typified by a reader/writer, using radio waves. As a type of these RFID tags, there has been proposed an RFID tag wherein a conductor pattern and an IC chip for radio communication are mounted on a base sheet. It has been conceived to use this type of RFID tag in such a manner that the tag is attached to an article or the like to, for example, identify the article by exchanging information about the article with an external apparatus.
There is a demand to reduce the size and weight of RFID tags and, in particular, to make them thinner, more flexible and less costly. In response to this demand, there has been proposed an RFID tag employing a film made of, for example, a resin material, such as polyethylene-terephthalate (PET), as a material of a substrate on which an IC chip is mounted (For example, see Japanese Patent Laid-Open No. 2001-156110).
FIG. 5 is a schematic view intended to explain a method of manufacturing an RFID tag in a conventional technique.
FIG. 5 sequentially shows in Parts (a) to (d) thereof respective steps for manufacturing an RFID tag.
In order to manufacture an RFID tag, there is first prepared a substrate 91 wherein a conductor pattern 912 functioning as an antenna of the RFID tag is formed on a film 911 made of PET, and then a thermoset adhesive 93p which is hardened by heating is attached onto this substrate 91, as shown in Part (a) of FIG. 5.
Next, as shown in Part (b) of FIG. 5, an IC chip 92 is mounted on the substrate 91 in a part to which the thermoset adhesive 93p has been attached. Bumps 921 to be connected to the conductor pattern 912 are formed on the IC chip 92. As shown in Part (c) of FIG. 5, the IC chip 92 is mounted on the substrate 91 so that the bumps 921 positionally align with the conductor pattern 912.
Next, as shown in Part (d) of FIG. 5, the substrate 91 whereon the IC chip 92 is mounted is clamped by a heating apparatus 8 from both the film 911 side and the IC chip 92 side of the substrate 91, with a supporting stage 82 and a heating head 81 of the heating apparatus 8, respectively. Then, the thermoset adhesive 93p is heated and hardened by the heating head 81 of the heating apparatus 8 abutting on the IC chip 92 side. In this way, the IC chip 92 is fixed to the substrate 91 with the bumps 921 in contact with the conductor pattern 912, thereby completing a small-sized lightweight RFID tag.
However, since the glass transition point of PET, which is the material of the film 911, is approximately 67° C. and, therefore, the upper temperature limit of PET is low, the film 911 is liable to deformation due to heating applied when hardening the thermoset adhesive 93p. 
FIG. 6 is a schematic view intended to explain the condition of the substrate in a heating step shown in Part (d) of FIG. 5.
If a heating treatment is carried out with the IC chip 92 mounted on the substrate 91 as shown in Part (a) of FIG. 6, the temperature of the substrate 91 rises and the film 911 becomes deformed, as shown in Part (b) of FIG. 6. If the thermoset adhesive 93p being hardened is fluidized along with the deformation of the film 911, air bubbles occur within the thermoset adhesive 93p and remain as voids 931 even after the thermoset adhesive 93p has hardened. The voids within the hardened thermoset adhesive 93p decrease adhesion strength between the IC chip 92 and the substrate 91, thereby degrading the reliability of the RFID tag.
The problem of reliability degradation due to the occurrence of such voids as described above is not limited to RFID tags but is common to electronic devices wherein a circuit chip is mounted on a film-like substrate.